1. Field of the Invention
The present invention relates generally to power supply circuits and power supply control methods therein, and more particularly to a power supply circuit and a power supply control method therein, the power supply circuit including a coil having supply voltage applied to one end thereof, a first switching element connected between the other end of the coil and ground, a second switching or diode element rectifying and supplying current to be supplied to a load in accordance with voltage generated at the connection of the coil and the first switching element, and a driver circuit switching at least the first switching element.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional power supply circuit 1.
Referring to FIG. 1, the power supply circuit 1 includes a coil L, a boost control circuit 11, and a Schottky barrier diode DO for rectification. A DC power supply 2 is connected to one end (a first end) of the coil L so that supply voltage Vcc is applied thereto. The boost control circuit 11 controls current flowing through the coil L. The Schottky barrier diode D0 rectifies current to a load 3 in accordance with voltage generated at the connection of the coil L and the boost control circuit 11, and supplies the current to the load 3.
The boost control circuit 11 includes an error amplifier 21, a comparator 22, an oscillator circuit 23, a driver 24, a transistor 25, and a power supply stop circuit 26. The boost control circuit 11 is driven by the supply voltage VCC supplied to a terminal T1 from the DC power supply 2.
The error amplifier 21 is connected to a terminal T3. The terminal T3 is connected to the connection of the load 3 and one end of a detection resistor Rs. The other end of the detection resistor Rs is grounded through terminals T4 and T5. The error amplifier 21 amplifies voltage at the connection of the load 3 and the detection resistor Rs, and supplies the amplified voltage to the comparator 22.
A detection signal from the error amplifier 21 and an oscillation signal from the oscillator circuit 23 are provided to the comparator 22. The oscillator circuit 23 generates a sawtooth or triangle wave oscillation signal. The comparator 22 compares the magnitudes of the detection signal provided from the error amplifier 21 and the sawtooth or triangle wave oscillation signal provided from the oscillator circuit 23. As a result, the comparator 22 generates a pulse signal whose pulse width increases as the detection signal level lowers and decreases as the detection signal level rises. The pulse signal generated in the comparator 22 is provided to the driver 24.
The driver 24 switches on and off the transistor 25 in accordance with the pulse signal provided from the comparator 22. The transistor 25 has a drain thereof connected to the other end (a second end) of the coil L through a terminal T2 and a source thereof grounded.
When the transistor 25 is switched on by the pulse signal provided from the driver 24, voltage at the terminal 2 is increased by the electromotive force of the coil L in accordance with the cycle of the pulse signal.
The connection of the second end of the coil L and the terminal T2 is connected to the load 3 via the Schottky barrier diode D0. The Schottky barrier diode D0 is connected in the forward direction toward the load 3 between the connection of the second end of the coil L and the terminal T2 and the load 3.
The voltage at the terminal T2 increased by the coil L is supplied to the load 3 via the Schottky barrier diode D0.
The load 3 is composed of, for instance, light emitting diodes D11 through D14. The light emitting diodes D11 through D14 are connected in series and caused to emit light by the increased voltage. As related art, a lighting device that causes a light emitting diode to emit light by voltage increased by a power supply circuit has been disclosed in Japanese Laid-Open Patent Application No. 2002-258363.
However, according to the conventional power supply circuit 1 shown in FIG. 1, the increasing of voltage, the supplying of the increased voltage to the load 3, and the operation of the load 3 are stopped by maintaining the transistor 25 in OFF state. At this point, the DC power supply 2 and the load 3 are connected via the Schottky barrier diode D0, which is connected in the forward direction with respect to the coil L and the supply voltage VCC. Therefore, when the transistor 25 is switched off, the supply voltage Vcc is applied to the load 3 through the Schottky barrier diode D0, so that a small current flows through the load 3.
This increases power consumption. Further, when the load 3 is a light emitting diode as in this case, there is a problem in that the light emitting diode emits light.